Defines a callback for a tessellation object.
OpenGL C bindings library: libGL.a
void gluTessCallback( GLUtesselator* tess, GLenum which, void (*CallBackFunc)())
The gluTessCallback subroutine defines a new callback for use by a tessellation object. If the specified callback is already defined, it is replaced. If the CallBackFunc parameter is set to null, the existing callback becomes undefined.
These callbacks are used by the tessellation object to describe how a polygon specified by the user is broken into triangles.
Note: There are two versions of each callback: one with user-specified polygon data and one without. If both versions of a particular callback are specified, then the callback with user-specified polygon data will be used. The polygon_data parameter used by some of the functions is a copy of the pointer that was specified when gluTessBeginPolygon was called.
The acceptable callbacks are as follows:
| GLU_TESS_BEGIN | The begin callback is invoked like glBegin to indicate the start of a
(triangle) primitive. The function takes a single argument of type
GLenum. If the GLU_TESS_BOUNDARY_ONLY property is set to
GL_FALSE, then the argument is set to either
GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, or
GL_TRIANGLES. If the
GLU_TESS_BOUNDARY_ONLY property is set to
GL_TRUE, then the argument will be set to
GL_LINE_LOOP. The function prototype for this callback is:
void begin(GLenum type); |
| GLU_TESS_BEGIN_DATA | The same as the GLU_TESS_BEGIN callback except
that it takes an additional pointer argument. This
pointer is identical to the opaque pointer
provided when gluTessBeginPolygon was called. The
function prototype for this callback is:
void beginData ( GLenum type, void *polygon_data ); |
| GLU_TESS_EDGE_FLAG | Similar to the glEdgeFlag subroutine. It takes a single
Boolean flag that indicates which edges lie on the polygon boundary.
If the flag is GL_TRUE, each vertex that follows
begins an edge that lies on the polygon boundary, that is, an edge that seperates an interior region from an exterior one. If the flag is
GL_FALSE, each vertex that follows begins an edge that kies in the polygon interior. To avoid confusion with the first few edges, the edge
flag callback is started before the first vertex callback is made.
Because triangle fans and strips do not support edge flags, the begin callback cannot be called with GL_TRIANGLE_FAN or GL_TRIANGLE_STRIP if the GLU_EDGE_FLAG If a non-null edge flag callback is provided. (If the callback is initialized to null, there is no impact on performance.) Instead, fans and strips are converted into independent triangles. The function prototype for this callback is: void edgeFlag(GLboolean flag); |
| GLU_TESS_EDGE_FLAG_DATA | The same as the GLU_TESS_EDGE_FLAG callback except that it
takes an additional pointer argument. This pointer is identical to the
opaque pointer provided when gluTessBeginPolygon was
called. The function prototype for this callback is:
void edgeFlagData(GLboolean flag, void *polygon_data ); |
| GLU_TESS_VERTEX | Started between the begin and end callbacks.
It is similar to the glVertex subroutine and defines the vertices of
the triangles created by the tessellation process. The function takes a pointer
as its only argument. This pointer is identical to the opaque pointer provided
by the user when the vertex was described. (See the
gluTessVertex
subroutine for details on specifying a polygon vertex.) The function prototype
for this callback is:
void vertex (void *vertex_data); |
| GLU_TESS_VERTEX_DATA | The same as the GLU_TESS_VERTEX callback except
that it takes an additional pointer argument. This
pointer is identical to the opaque pointer
provided when gluTessBeginPolygon was called. The
function prototype for this callback is:
void vertexData ( void *vertex_data,
void *polygon_data ); |
| GLU_TESS_END | Serves the same purpose as the glEnd subroutine and
indicates the end of a primitive. It takes no arguments.
The function prototype for this callback is:
void end(void); |
| GLU_TESS_END_DATA | The same as the GLU_TESS_END callback except that
it takes an additional pointer argument. This
pointer is identical to the opaque pointer
provided when gluTessBeginPolygon was called. The
function prototype for this callback is:
void endData ( void *polygon_data); |
| GLU_TESS_COMBINE | The combine callback is called to create a new
vertex when the tessellation detects an
intersection, or wishes to merge features. The
function takes four arguments: an array of three
elements each of type GLdouble, an array of four
pointers, an array of four elements each of type
GLfloat, and a pointer to a pointer. The prototype
Is:
void combine( GLdouble coords[3],
void *vertex_data[4],
GLfloat weight[4],
void **outData );
The vertex is defined as a linear combination of up to four existing vertices, stored in vertex_data. The coefficients of the linear combination are given by weight; these weights always add up to 1. All vertex pointers are valid even when some of the weights are 0. coords gives the location of the new vertex. The user must allocate another vertex, interpolate parameters using vertex_data and weight, and return the new vertex pointer in outData. This handle is supplied during rendering callbacks. The user is responsible for freeing the memory some time after gluTessEndPolygon is called. For example, if the polygon lies in an arbitrary plane in 3-space, and a color is associated with each vertex, the GLU_TESS_COMBINE callback might look like this: void myCombine( GLdouble coords[3], VERTEX *d[4],
GLfloat w[4], VERTEX **dataOut )
{VERTEX *new = new_vertex();
new->x = coords[0];
new->y = coords[1];
new->z = coords[2];
new->r = w[0]*d[0]->r + w[1]*d[1]->r +
w[2]*d[2]->r + w[3]*d[3]->r;
new->g = w[0]*d[0]->g + w[1]*d[1]->g +
w[2]*d[2]->g + w[3]*d[3]->g;
new->b = w[0]*d[0]->b + w[1]*d[1]->b +
w[2]*d[2]->b + w[3]*d[3]->b;
new->a = w[0]*d[0]->a + w[1]*d[1]->a +
w[2]*d[2]->a + w[3]*d[3]->a;
*dataOut = new; }
If the tessellation detects an intersection, then the GLU_TESS_COMBINE or GLU_TESS_COMBINE_DATA callback (see below) must be defined, and it must write a non-NULL pointer into dataOut. Otherwise the GLU_TESS_NEED_COMBINE_CALLBACK error occurs, and no output is generated. |
| GLU_TESS_COMBINE_DATA | The same as the GLU_TESS_COMBINE callback except
that it takes an additional pointer argument. This
pointer is identical to the opaque pointer
provided when gluTessBeginPolygon was called. The
function prototype for this callback is:
void combineData ( GLdouble coords[3],
void *vertex_data[4],
GLfloat weight[4],
void **outData,
void *polygon_data ); |
| GLU_TESS_ERROR | Called when an error is encountered. Character strings describing these errors
can be retrieved with the gluErrorString subroutine. The GLenum type is the
only argument and indicates the specific error that occurred. It will be set to one of the following:
Note: The GLU library will recover from the first four errors by inserting the missing call(s). The function prototype for this callback is: void error(GLenum errno); |
| GLU_TESS_ERROR_DATA | The same as the GLU_TESS_ERROR callback except
that it takes an additional pointer argument. This
pointer is identical to the opaque pointer
provided when gluTessBeginPolygon was called. The
function prototype for this callback is:
void errorData ( GLenum errno, void *polygon_data ); |
| tess | Specifies the tessellation object created with the gluNewTess subroutine. |
| which | Specifies the callback being defined. The following values are valid: |
| CallBackFunc | Specifies the new callback. |
Tessellated polygons can be rendered directly as in the following example:
gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin);
gluTessCallback(tobj, GLU_TESS_VERTEX, glVertex3dv);
gluTessCallback(tobj, GLU_TESS_END, glEnd);
gluTessCallback(tobj, GLU_TESS_COMBINE, myCombine);
gluTessBeginPolygon(tobj, NULL);
gluTessBeginContour(tobj);
gluTessVertex(tobj, v, v);
...
gluTessEndContour(tobj);
gluTessEndPolygon(tobj);
Typically, the tessellated polygon should be stored in a display list so that is does need to be retessellated every time it is rendered.
| /usr/include/GL/gl.h | Contains C language constraints, variable type definitions, and ANSI function prototypes for OpenGL. |
The glBegin subroutine, glEdgeFlag subroutine, glVertex subroutine, gluNewTess subroutine, gluTessVertex subroutine, gluErrorString subroutine, gluTessBeginPolygon subroutine, gluTessBeginContour subroutine, gluTessProperty subroutine, gluTessNormal subroutine.